I'm afraid I don't have the opportunity to expand on this much, but you might take a look at Robert Bartlet's The Making of Europe: Conquest, Colonization, and Cultural Change, 950-1350. He argues that from a "core" region in France, England, the Rhineland, and northern Italy, medieval societies effectively colonized the rest of Europe: the Celtic fringe, Muslim Iberia, Scandinavia, and eastern and southern Europe. This included campaigns of conquest at the conclusion of which aristocratic leaders frequently settled in their conquered territories and assimilated, as well as a kind of internal colonization in which peasants were given incentives to simply build new settlements on unoccupied lands.

Of course, the book is more than 20 years old and I'm sure his arguments have since been disputed, but it might be a good place for you to start.